Lubricant oils have been used to lubricate internal combustion engines, devices in driving systems (e.g., automatic transmissions, shock absorbers and power steerings) and gears having sliding mechanical parts for their smooth operation. In particular, lubricant oil for internal combustion engines are used mainly for piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve trains including cams and valve lifters, and other sliding mechanical parts. They are also used for cooling the engines, cleaning and dispersing combustion products, and preventing rust and corrosion, in addition to the lubricating purposes. As described above, lubricant oils for internal combustion engines are required to exhibit a variety of functions. These requirements are becoming even severer, as the engines become more functional, produce higher power and are operated under severer conditions.
In order to satisfy these requirements, lubricant oils for internal combustion engines are incorporated with a variety of additives, such as antiwear agent, metallic detergent, ashless dispersant and antioxidant. The essential functions of a lubricant oil for internal combustion engines are to prevent wear and seizure by helping the engine operate smoothly under all conditions. Hydrodynamic lubrication prevails in lubricated engine mechanical parts, but boundary lubrication tends to occur in some sections, e.g., valve trains and dead centers in the cylinders. In general, zinc dithiophosphate or the like is added to prevent wear in the boundary lubrication areas.
More recently, abatement of CO.sub.2 emissions has been actively pursued, in order to prevent global warming. It is necessary in Japan to improve fuel economy of diesel engines by 14.9% on the average over the 1995 level from 2005 on, which also urges to develop fuel-saving characteristics of diesel engine oils.
An internal combustion engine loses energy greatly in the frictional sections for which a lubricant oil is used. Therefore, a lubricant oil for internal combustion engines is incorporated with a combination of various additives, e.g., friction modifier, in order to reduce friction losses and fuel consumption (as disclosed by, e.g., Japanese Patent Publication No. 3-23595). Internal combustion engines are operated under widely varying conditions with respect to oil temperature, rotational speed and load, and a lubricant oil for internal combustion engines is required to exhibit low friction coefficient under widely varying service conditions, in order to further improve fuel efficiency.
A variety of techniques have been proposed to reduce friction coefficient of lubricating oil, e.g., incorporation of various additives, such as an organomolybdenum compound, and combination of organomolybdenum compound and metallic detergent (e.g., Japanese Patent Publication No. 6-62983, abstract); and combination of organomolybdenum compound and sulfur-based compound (e.g., Japanese Patent Publication No. 5-83599, abstract), and combination of organomolybdenum compound, zinc dithiophosphate and sulfur-based compound (e.g., Japanese Laid-open Patent Application No. 8-73878 EP699-739). The examples of friction reducing agents other than an organo-molybdenum compound include a combination of a partial ester of fatty acid with glycerol and organocopper compound (e.g., Japanese Patent Publication No. 3-77837, abstract), and a combination of a pentaerythiritol ester and succinimide or zinc dithiophosphate (e.g., Japanese Laid-open Patent Application No. 55-80494 CA1136606 and 55-82195 U.S. Pat. No. 4,584,112).
However, in case of a diesel engine, unlike gasoline engine, the engine oil tends to be contaminated with large quantities of soot generated as a result of incomplete combustion of diesel fuel oil. It is reported that the soot, having surface activity, may adsorb a polar additive in the engine oil and scrape off a film layer formed on the rubbing surface. The required functions of a friction-reducing agent for diesel engines, therefore, should be much different from those of the agent for gasoline engines under the severe friction conditions with the engine oil contaminated with soot. Therefore, the conventional friction- reducing agents, e.g., organomolybdenum compound, amine, amide and phosphate ester, may not sufficiently improve fuel economy. Only a limited number of proposals have been made to improve fuel economy of diesel engines, including incorporation in the base oil of hydrated borate of an alkali metal (e.g., Japanese Patent Publication No. 1-48319, abstract).
Air pollution by exhaust gases (in particular, NOx) from diesel engines is becoming more severe worldwide, and there are movements to introduce more stringent regulations on NOx and particulate matter emissions from diesel engines. Engine makers are responding to these regulations by an EGR system, which is already adopted for gasoline engines, to clear the NOx regulations. Some of the problems involved in use of an EGR system are still more increased quantities of soot in the lubricant oil to aggravate wear of valve trains and piston-cylinder interfaces, and to prevent the friction-reducing agent from fully exhibiting its inherent characteristics of improving fuel economy. Moreover, it should be noted that abatement of NOx and particulate matter run counter to each other, when an EGR system is adopted for NOx abatement. One of the methods trying to solve problems of increased particulate matter in an EGR-equipped engine is use of high-pressure fuel injection, where high-pressure fuel is stored in a pressure-accumulating piping system (referred to as common rail) by means of a fuel supply pump and then injected into each engine cylinder under pressure from the common rail via a valve, to improve combustion conditions. It is considered to be important that the future diesel engine must be equipped with an EGR system and pressure-accumulating type fuel injector simultaneously to clear the more stringent exhaust gas regulations. This pressure-accumulating type injector will greatly improve combustion conditions, thereby reducing contamination of the oil with soot.
In spite of these improvements of the engine side, the conventional friction-reducing agent compounding techniques have failed to give the agent which allows the lubricating oil composition to improve fuel economy of diesel engines for extended periods.
It is also known that reducing viscosity of engine oil is one of the effective means to improve fuel economy, and multi-grade diesel engine oils with low-viscosity base oils incorporated with a viscosity index improver, e.g., polymethacrylate and ethylene-propylene copolymer, have been generally used.
However, the effect of improving fuel economy by a multi-grade diesel engine oil incorporated only with a viscosity index improver is far from sufficient. Therefore, there are strong demands for more advanced lubricant oil compositions which exhibit satisfactory effects of improving fuel economy for diesel and gasoline engines stably for extended periods.